Magnetically actuated friction damper

ABSTRACT

A friction damper with a housing including a cavity. The damper includes a first member disposed in the cavity and movable in the cavity; a second member disposed in the cavity; an intermediate member between the first and second members, the first member being in frictional engagement with the intermediate member; and at least one magnetic field generator mounted to magnetically couple the first and second members thereby maintaining the first member in frictional engagement with the intermediate member and wherein the first member is movable against the intermediate member to generate a damping force.

This application is a divisional of U.S. patent application Ser. No.09/737,889, filed Dec. 15, 2000, which is now U.S. Pat. No. 7,040,467.

FIELD OF INVENTION

The invention relates to a passive damper, and more particularly theinvention relates to a friction damper where the normal force of thefriction is provided by magnetic attraction between first and seconddamping members.

BACKGROUND OF INVENTION

Friction dampers generally apply a frictional force to a moveable memberto dissipate translational or rotational energy of the member to produceacceptable member movement.

Prior art frictional dampers are typically comprised of surface effectdampers of the type described in U.S. Pat. No. 5,257,680 to Corcoran etal., and U.S. Pat. No. 4,957,279 to Thorn. Surface effect dampersoperate by dissipating translational or rotational energy by working anelastomeric element to convert kinetic energy to heat. Such conventionaldampers are generally comprised of a housing with an inner wall, and anelastomeric member movable through the housing. Interference between theinner wall and the elastomeric member produces the friction damping.

Additionally, frictional damping may be supplied to a movable member bya friction damper that utilizes a controllable fluid to preciselycontrol the supplied damping force. Such devices are well known in theart as magnetorheological (MR) fluid devices and examples of MR devicescan be found in commonly assigned U.S. Pat. No. 5,284,330 to Carlson etal.; and U.S. Pat. No. 5,277,281 also to Carlson et al. MR devices maybe of the rotary or linear acting variety and such dampers employ acontrollable MR fluid comprised of fine soft-magnetic particlesdisbursed within a liquid carrier. MR fluids exhibit a “thickening”behavior (a rheology change) sometimes referred to as an apparentviscosity change upon being exposed to a magnetic field of sufficientstrength. The higher the magnetic field strength exposed to the MRfluid, the higher the damping force that can be achieved with aparticular MR device. Although effective in providing damping in a largenumber of applications, conventional surface effect and MR frictiondampers have a number of shortcomings. First, prior art dampers aresensitive to temperature changes and thermal expansion. When the priorart dampers are subjected to significant temperature increases ordecreases the viscosity of the MR fluid may be affected and the changein fluid viscosity may in turn affect the supplied damping force. Suchtemperature changes can also affect the properties of the elastomerdamping element and can cause the elastomeric damping element tocontract or expand and experience dimensional changes. Changes to thedamping element dimensions or properties will change the damping forcessupplied by the surface effect friction damper.

Surface effect damping is provided by a carefully calculatedinterference, between the housing and elastomer element. In MR deviceseffective damping is ensured by maintaining a precisely defined gapdimension between the housing and piston member. The MR fluid flowsthrough the defined gap. As a result of the foregoing, prior art dampersare very sensitive to dimensional tolerancing and tolerances must betightly maintained in order for prior art friction dampers to provideeffective damping forces. However, overtime, through repetitive use ofthe dampers, the critical tolerances between moving damper componentsare frequently lost and the deviations in the part tolerances negativelyaffects the forces provided by the friction damper. Finally, prior artfriction dampers can be difficult to assemble and only a specific rangeof materials are acceptable for use in such prior art friction dampers.

The foregoing illustrates limitations known to exist in present devicesand methods. Thus, it is apparent that it would be advantageous toprovide an alternative directed to overcoming one or more of thelimitations set forth above. Accordingly, a suitable alternativefriction damper is provided including features more fully disclosedhereinafter.

SUMMARY OF THE INVENTION

This is accomplished by the present invention that provides a frictiondamper that provides effective damping forces without sensitivity totemperature changes or tolerances between component parts; is easilyassembled and may incorporate components made from a variety ofmaterials.

In one aspect of the present invention this is accomplished by thefriction damper of the present invention. The damper comprises a housingincluding a cavity formed therein; a first member disposed in saidcavity and movable in said cavity; a second member disposed in saidcavity; an intermediate member between the first and second members, thefirst member being in frictional engagement with the intermediatemember; and at least one magnetic field generator mounted tomagnetically couple the first and second members thereby maintaining thefirst member in frictional engagement with the intermediate member andwherein the first member is movable against the intermediate member togenerate a damping force.

The second member may be movable with the first member. Before the firstmember is displaced, the first and second members are aligned. When thefirst member is initially displaced, the second member lags behind thefirst member by a distance, and then is drawn towards the first memberso that the first and second members are substantially aligned.Alternatively, the second member may be fixed. In both instances thefirst and second members are coupled magnetically and as a result,stiction between moveable damper members is eliminated by the damper ofthe present invention which provides for a smooth onset of dampingforce.

Additionally, in an alternate embodiment the first and second membersand intermediate members may not be located in housing. In such anembodiment the ends of the intermediate member are fixed.

The foregoing and other aspects will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the friction damper of the presentinvention with the housing endcap removed;

FIG. 2 is a longitudinal sectional view of the damper of FIG. 1;

FIG. 3 is a lateral sectional view taken along line 3-3 of FIG. 1;

FIG. 4 is the longitudinal section view of FIG. 1 illustrating thealignment of the first and second members and intermediate member beforethe first member is moved;

FIG. 5 is the longitudinal section view of FIG. 4 after the first memberis displaced;

FIG. 6 is a plot of Force versus Displacement for a prior art frictiondamper;

FIG. 7 is a plot of Force versus Displacement for the friction damper ofthe present invention;

FIG. 8 is a longitudinal section view of a second embodiment frictiondamper of the present invention;

FIG. 9 is a lateral sectional view taken along line 9-9 of FIG. 8;

FIG. 10 is a longitudinal section view of a third embodiment frictiondamper of the present invention;

FIG. 11 is a longitudinal sectional view of the first member, secondmember, intermediate member and bearing layer of a fourth embodiment ofthe friction damper of the present invention; and

FIG. 12 is a longitudinal sectional view of an alternate embodiment ofthe present invention with the first and second members and intermediatemember unenclosed by a housing with the ends of the intermediate memberfixed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now turning to the drawing figures wherein like parts are referred to bythe same numbers in the several views, FIGS. 1-5 disclose a firstembodiment friction damper 10.

Friction damper 10 includes an elongate, tubular housing 12 with housingwall 14 that defines an inner housing surface 16 and housing cavity 17.The housing is most preferably made of a non-metallic material such asplastic and although the housing is shown and described as being tubularwith a circular cross-section, it should be understood that the housingmay have any suitable cross section such as a rectangular or squareconfiguration for example.

The housing includes first and second housing ends 18 and 20respectively and the ends are closed by respective first and second endcaps 22 and 24. At least one of the end caps is removably attached toits respective housing end. For purposes of describing the preferredembodiment of the present invention, end cap 24 is fixed to secondhousing end 20 and end cap 22 is removably attached to first housing end18 by a threaded connection, interference fit or other conventionalremovable attachment means. End cap 22 includes hole 26 through whichshaft 28 extends outwardly from the housing cavity so that the shaft end30 may be connected to a movable component of a toy, haptic device,automobile door or appliance such as a washing machine, for example.Bracket 32 is made integral with end cap 24 and the bracket serves as ameans for anchoring the second housing end during operation of thedamper 10.

Longitudinally extending opposed slots 34 and 36 are provided along theinner surface of housing wall 14 and the slots open at the housing ends18 and 20. Slots 34 and 36 are adapted to receive the longitudinal edgesof an intermediate member 40. As the description proceeds theintermediate member may also be referred to as an intermediate frictionlayer. With the longitudinal edges of member 40 located in slots 34 and36 member 40 extends diametrically across the cavity 17 to divide thelarger cavity into smaller first and second cavities 17 a and 17 brespectively. See FIGS. 1 and 3. The intermediate friction layer 40 maybe made from a variety of non-magnetic materials such as polyethylene orother plastics, stainless steel, brass, laminates or composite materialsused in brake pads for automotive applications.

The friction damper 10 of the present invention is insensitive to wearand dimensional tolerances. Variations in the thickness of the frictionlayer 40 due to wear or loose manufacturing tolerances have littleeffect on damper performance because the normal frictional force loadingof the first and second frictional elements 50 and 60 is not determineddimensionally as it is in other friction dampers such as surface effectdampers for example. Thus, even if a large portion of the intermediatemember 40 were to be worn away over time, the normal loading forcebetween the elements 50 and 60 would not be affected.

The first member 50 is located in cavity 17 a and is connected to oneend of shaft 28 in a conventional manner. The shaft is connected to thenonmagnetic seat 52 of member 50 and the seat in turn supports a pair ofpermanent magnets 54 a, 54 b that are located side-by-side in the seat.The seat may be made of any suitable nonmagnetic material such as aplastic or aluminum for example. As shown in FIGS. 2 and 3, when themember 50 is located in cavity 17 a, the seat is at all times located onand in frictional engagement with the intermediate member 40 and themagnets 54 a, 54 b are prevented from becoming unseated by a metallickeeper plate 56 that is attracted to the magnets. In addition topreventing magnet displacement, the plate 56 closes magnetic circuit 70to produce a reliable and consistent magnetic field. As the shaft 28 ismoved axially in either of the directions identified by arrows 100 and102, the member 50 is moved in the same axial direction and the seat andmagnets are maintained in frictional engagement with the member 40.

The second member 60 is located in cavity 17 b and is magneticallycoupled to member 50. The magnetic coupling is represented by the dashedfont representation of magnetic field 70. Like first member 50, secondmember 60 includes side-by-side magnets 64 a, 64 b that are supported inseat 62. A keeper plate 66 covers the magnets along one side of theseat. As shown in FIG. 2 the magnets 54 a, 54 b and 64 a, 64 b aresupported in their respective seats so that their north and south polesare oriented in the manner required to produce the coupling magneticfield 70. The keeper plates 56 and 66 complete the magnetic couplingcircuit. In this way, the members 50 and 60 are mutually attractedthrough the intermediate member 40. When member 60 is located in cavity17 b, seat 62 is at all times in frictional engagement with member 40.As will be described in greater detail hereinbelow, second member 60moves in the general directions identified by arrows 100 and 102, andfollows the movement of first member 50.

It should be understood that the friction damper of the presentinvention does not need to be in the specific horizontal orientationdisclosed for the damper to provide effective damping. Although in FIG.3 cavity 17 a is shown as an upper cavity and cavity 17 b is shown as alower cavity, the orientations of cavities 17 a and 17 b could bereversed with the cavity 17 b as the upper cavity and cavity 17 a as thelower cavity or the housing could be rotated to any angle from theposition of FIG. 3.

Operation of friction damper 10 will now be described. For simplicity asthe description proceeds the operation of damper 10 will be described interms of movement of member 60 in direction 100. However it should beunderstood that the damper 10 operates in the same way if moved indirection 102 shown in FIG. 2.

After the damper is assembled, end 24 is fixed via bracket and shaft end30 is connected to a movable object of interest. When the damper isassembled the first and second members 50 and 60 are magneticallycoupled and are maintained against the surface of member 40 by magneticfield 70.

Turning now to FIGS. 4 and 5, the second member 60 tends to align withthe first member 50 due to the magnetic attraction between magnets 54 a,54 b and 64 a, 64 b as shown in FIG. 4. As the first member movesaxially along intermediate member 40 in direction 100, the member 60tends to lag behind or become partially decoupled from the first member50. See FIG. 5. The lag of member 60 is represented by distance X inFIG. 5. This is because the members 50 and 60 are effectively coupled bya magnetic spring. Although the shaft 28 is directly coupled to thefirst member, the second member 60 is effectively coupled to themagnetic spring. Thus before the second member begins to follow themovement of the first member the magnetic spring must first be displacedsufficiently to provide enough force to overcome the friction of thefirst element. Once the second member overcomes the friction of thefirst element, the second member 60 is rapidly drawn back towards thefirst member until the first and second members are substantiallyaligned. As the first member continues to be displaced, the secondmember lags behind the first member by a relatively small lag distanceY. See FIG. 4.

This partial decoupling of the first and second members provides asignificant advantage in terms of the “feel” of the damper. The partialdecoupling ameliorates much of the stick-slip normally associated with afriction damper. Such stick-slip is referred to by those skilled in theart as stiction. The effects of stiction in prior art friction dampersare illustrated graphically in FIG. 6. The damping force supplied by aprior art damper initially is high as a result of the resistance tomovement because of the presence of stiction and then once the dampingelement overcomes the forces of stiction at point A of FIG. 6, thedamper force is lowered to its operating level. FIG. 7 graphicallyillustrates the damper of the present invention that provides a smooth,stiction-free increase in force until the damper force reaches itsworking level. See the point identified as B in FIG. 7.

A second embodiment friction damper 200 is illustrated in FIGS. 8 and 9.The second embodiment friction damper 200 comprises all of the elementscomprising friction damper 10 except for the movable second member 60.The second embodiment friction damper includes a stationary secondmember 205 that spans the longitudinal dimension of the housing. Thesecond member is located in the chamber with the lower member edgeslocated supported on the inner surface of the housing wall. See FIG. 9.

A third embodiment friction damper 300 is illustrated in FIG. 10. Thethird embodiment friction damper includes all of the elements of thesecond embodiment friction damper 200 except for an intermediate member210 that extends the longitudinal length of the housing. Rather, in thethird embodiment friction damper 300, the intermediate member isattached to the underside of the first seat 52 to be moveable with theseat as member 50 is moved relative to the second member. The member 210is attached by an adhesive or other conventional means. In an alternateembodiment, the friction damper could include both the friction layer210 along the underside of the seat 52 and intermediate member 40 alongthe complete length of the second member 205.

A fourth embodiment friction damper 400 is illustrated in FIG. 11. Thefriction damper includes the features of the second embodiment frictiondamper 200 and also includes a bearing member 405 that is made of arubber or other resilient material, and the member 405 is fixed to theunderside of the seat 52 in a conventional manner. The bearing memberincludes a plurality of spaced apart semispherical ribs 410 that extendlaterally in a parallel manner across the member. The ribs remain infrictional engagement with the intermediate member during operation ofthe damper 400.

In the second, third and fourth embodiments, the first member 50 ismagnetically coupled with the second member 205. The second member is asoft magnetic layer such as iron or steel. In friction dampers 200, 300and 400 as the first member is moved longitudinally, the smooth stictionfree behavior of FIG. 7 is achieved with a smooth increase in the forcesupplied by the dampers 200, 300 and 400.

In the first through fourth embodiments 100, 200, 300 and 400 the firstand second members 50 and 60 and intermediate friction layer 40 may beunenclosed by housing 12. Such a configuration is shown in FIG. 12. Insuch an alternate configuration, the ends of intermediate member 40 aremaintained fixed by attachment members 500 and 502 which may be anysuitable attachment member such as a bracket or the like. An alternativeto the fixed member ends includes fixing portions of the longitudinaledges of the member 40 in such a manner that does not interfere with thedisplacement of members 50 and 60.

While I have illustrated and described a preferred embodiment of myinvention, it is understood that this is capable of modification, andtherefore do not wish to be limited to the precise details set forth,but desire to avail myself of such changes and alterations as fallwithin the purview of the following claims.

1. A damper comprising; a housing including a cavity formed therein; afirst movable member disposed in said cavity and movable in said cavity;a second stationary member disposed in said cavity; an intermediatemember between the first movable member and the second stationarymember, the intermediate member being in frictional engagement with thesecond stationary member; and at least one magnetic field generatormounted to magnetically couple the first and second members therebymaintaining the second member in frictional engagement with theintermediate member and the intermediate member is movable against thesecond member to generate a damping force with the intermediate memberfixed to the first member.
 2. The damper as claimed in claim 1 whereinthe first member comprises a first seat, the at least one magnetic fieldgenerator being supported in the first seat, the first member furthercomprising a keeper to maintain the at least one magnetic fieldgenerator in the first seat.
 3. The damper as claimed in claim 2 whereinthe at least one magnetic field generator of the first member iscomprised of a plurality of permanent magnets.
 4. The damper as claimedin claim 1 wherein the housing is comprised of a tubular member having awall defining an inner housing support surface for supporting the secondmember, the housing further comprising first and second end cap members.5. The damper as claimed in claim 4 wherein the inner housing supportsurface for supporting the second member are comprised of opposedlongitudinally extending slots.
 6. A damper comprising: a housing with ahousing wall and including a cavity formed therein; a first movablemember disposed in said cavity and movable in said cavity; a secondstationary member disposed in said cavity; an intermediate memberbetween the first movable member and the second stationary member, thefirst member being in frictional engagement with the intermediatemember; and at least one magnetic field generator mounted tomagnetically couple the first and second members thereby maintaining thefirst member in frictional engagement with the intermediate member, thefirst member is movable against the intermediate member to generate adamping force, with a friction layer between the intermediate member andthe first member, the friction layer comprises a plurality of ribs andthe second member is supported by the housing wall.